Most speed 400 Sokols are running on 7 and 8 cell 600 AE's.
I fly mine on 7 cells with a 6 volt speed 400.
Performance is great and run time is 3-4 minutes. Don't be disapointed with the time as it is a very busy 3-4 minutes.
I don't know how big a 700AR cell is so I can't help you there. You do want to keep the plane light.